Fetal growth restriction (FGR) complicates 4 to 7% of pregnancies in developed countries, and is a major contributor to perinatal morbidity and mortality. Inadequate uteroplacental perfusion is the most commonly recognized cause of FGR. Placental perfusion is largely regulated by locally acting vasoactive mediators. The purpose of the proposed research is to investigate the mechanisms by which two vasoactive mediators contribute to the pathogenesis of FGR. The focus will be on endothelin-1 (ET-1) and platelet activating factor (PAF), both potent vasoconstrictors. There is evidence to suggest that ET-1 and PAF may act synergistically to decrease perfusion under pathologic conditions. The mechanisms by which ET-1 and PAF interact in the pathophysiology of FGR will be investigated. ET-1 contributes to the regulation of uterine and placental vascular tone. ET-1 has been shown to play a primary role in the pathophysiology of three different models of FGR. Beyond its direct vasoconstrictive effects, ET-1 also upregulates the production of PAF, a potent vasoactive and inflammatory mediator. Endothelin receptor A (ETA) antagonism improves placental perfusion and fetal growth in these models. In preliminary experiments, we demonstrated that placental perfusion is further improved with a more highly selective ETA antagonist. In nitric oxide synthase (NOS) inhibition-induced FGR, the effect of three different antagonists, which vary with respect to their degree of ETA receptor selectivity, on placental perfusion, fetal growth, and PAF expression will be compared. In this way the degree to which ETA regulates PAF production, uteroplacental perfusion, and fetal growth will be evaluated. eNOS knockout (KO) mice will also be used to evaluate the expression of ET-1, ET receptors, and PAF in the uterus and placenta in a model of NO deficiency without chemical intervention. This will allow further evaluation of the effect of both ETA and PAF antagonism on fetal and placental growth and vasoactive mediator expression. To investigate the molecular regulation of these mediators, human placental trophoblasts will be utilized with a goal of understanding signaling mechanisms that regulate the production and action of both ET-1 and PAF in the placenta. These studies will provide new insights on how these prominent vasoactive mediators contribute to the pathophysiology of FGR. A better understanding of these mechanisms may allow for the development of meaningful therapeutic interventions to improve outcome in pregnancies complicated by FGR, particularly those remote from term. [unreadable] [unreadable]